Infrared and Visible Dielectric Function of Electroplated Bi[sub 2±x]Te[sub 3±x] Films Determined by Spectroscopic Ellipsometry

The pseudodielectric functions of electroplated thin films with atomic bismuth content varying from 1.8 to 2.2 were determined using spectroscopic ellipsometry in the energy range of 0.03–3.10 eV. The experimental dielectric functions show differences with the literature bulk single-crystal data which may be explained by a ca. 50% porosity in the electroplated films. In the visible range, the optical constants did not seem to depend on the film composition while in the infrared range a rough tendency can be established: the real part of the dielectric function shifted to lower values with increasing bismuth content while the imaginary part shifted toward higher values. In the infrared range, Tauc–Lorentz combined with Drude dispersion relations were successfully used. The energy bandgap was found to be about 0.11 eV independent of the film composition. This result is related to the minor evolution of the film composition. Moreover, the fundamental absorption edge was described by an indirect optical band-to-band transition. The thickness of the thinner films were estimated, which are comprised between 1.77 and following the samples. Electrical resistivity and energy-independent scattering time were found to be related to the bismuth content.